(a) Technical Field
The present invention relates to a fuel cell stack. More particularly, it relates to a fuel cell stack with improved corrosion resistance, in which the outer edge of the fuel cell stack including an outer cut portion of each metallic bipolar plate can be effectively prevented from being corroded.
(b) Background Art
A fuel cell is an electrical generation system that does not convert chemical energy of fuel into heat by combustion, but rather electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. A fuel cell can be applied to the electric power supply of small-sized electrical and electronic devices, for example, portable devices, as well as industrial and household appliances and vehicles.
One of the most attractive fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which has the highest power density among other fuel cells. The PEMFC has a fast start-up time and a fast reaction time for power conversion due to its low operation temperature.
As shown in FIG. 1, a fuel cell stack 10 included in the PEMFC has a membrane-electrode assembly (MEA) 11, in which an electrolyte/catalyst layer (where an electrochemical reaction takes place) is disposed on each of both sides of a polymer electrolyte membrane through which hydrogen ions are transported, a gas diffusion layer (GDL) 12 which functions to uniformly diffuse reactant gases and transmit generated electricity, a gasket 14 and a sealing member (not shown) which functions to provide an appropriate airtightness to reactant gases and coolant and to provide an appropriate bonding to pressure, and a bipolar plate 13 which functions to transmit reactant gases and coolant.
When the fuel cell stack 10 is assembled with the unit cells, a combination of the MEA 11 and the GDL 12 is positioned in the center of each unit cell of the fuel cell stack. The MEA 11 has a cathode and an anode as the electrode/catalyst layer, in which an electrochemical reaction between hydrogen (fuel) and oxygen (oxidant) takes place, disposed on each of both sides of the polymer electrolyte membrane. Moreover, the GDL 12 and the gasket 14 are sequentially stacked on both sides of the MEA 11, where the cathode and the anode are located.
Further, a metallic bipolar plate has been developed to substitute an existing graphite bipolar plate, which consumers a considerable portion of production cost. This metallic bipolar plate allows for mass production by providing an increase in production rate as well as a reduction in the overall production cost.
The metallic bipolar plate can be made from many different metallic materials such as steel, stainless steel, aluminum, etc. Typically, however, the metallic bipolar plates are made from stainless steel.
However, metallic materials are known for having low corrosion resistance, which in turn can affect the stability of electrical conductivity. One known method of reducing or preventing the amount of corrosion is to apply an anti-corrosion coating using a precious metal.
However, even when the anti-corrosion coating is applied, an outer cut portion of the metallic bipolar plate is vulnerable to corrosion, and since the humidified gas is supplied into the fuel cell and water is produced as a by-product of the electrochemical reaction, the metallic bipolar plate is continuously exposed to corrosive environments.
Some previous designs have used an enclosure surrounding the outside of the fuel cell stack to protect the fuel cell stack from water vapor, dust, vacuum, etc., applied from external environments of the vehicle. However, this enclosure can only protect the fuel cell stack from external corrosion factors and, when the water present in the fuel cell stack (such as bipolar plate channels, electrolyte membranes, electrodes, GDLs, etc.) leaks to the outside due to unexpected conditions, the enclosure prevents the water vapor in the enclosure from being discharged to the outside, which may accelerate the corrosion of the metallic bipolar plate. For example, when the airtightness is lost due to damage of the gasket or when the claiming force for clamping the bipolar plate is reduced, the internal water vapor may leak to the outside through a gap in the gasket, which causes serious corrosion to the metallic bipolar plate. Since corrosion of the bipolar plate is associated with the overall safety of the fuel cell and it is thus important to prevent the corrosion as far in advance as possible.